Muddle mint leaves (about 5-8 will do) into a couple of teaspoons of sugar and a good squeeze of lime juice (2 tablespoons for the purists). You want to really release the oils from the mint to give you a big hit of flavour.

Add as much rum as you prefer and pour into a glass filled halfway with crushed ice. Top up with soda water (and a squeeze of grapefruit if you have it). Garnish with some extra mint leaves and a lime wedge if you like.